This invention relates to the acquisition and display of medical ultrasonic image data. In particular, the present invention relates to altering the dynamic range used to process image data, as a function of noise actually detected in scanning particular regions of a patient""s body, to achieve the best possible results in imaging the patient.
Ultrasonic imaging technology has been a tremendous boon to the medical field. Ultrasonic imaging allows physicians to examine patients"" internal tissues and organs without resorting to the use of ionizing radiation or invasive exploratory surgery. As a result, ultrasonic imaging is a very important diagnostic technology for, among many other applications, reviewing fetal development and recovery from injuries which require frequent internal examinations.
As is well known, in ultrasonic imaging, a series of high-frequency sonic pulses are generated, and these pulses xe2x80x9cbouncexe2x80x9d off various objects in their path. Specifically, different structures in a patient""s body exhibit different levels of impedance, and ultrasonic echoes are generated when the ultrasonic signals contact impedance boundaries between these structures. The interval between the emission of the pulses and the receipt of the corresponding echoes is measured to determine the distance between the source of the pulse and the impedance boundary from which the echo resulted. In addition, the relative intensity of the echo conveys information regarding the nature of the tissues causing the echoes. Different tissues exhibit different levels of impedance to the ultrasonic signals. Therefore, varying impedance differentials exist, for example, at the boundary between muscle tissue and bone as opposed to the boundary between fatty tissue and bone. As a result, when an ultrasonic pulse strikes the impedance boundary between muscle tissue and bone, a more robust echo is generated than the echo generated when an ultrasonic pulse strikes the impedance boundary between fatty tissue and organ tissue. Ultimately, it is the mosaic assembled from each of these echoes received, reflecting the position and the nature of the objects causing the echoes, that constitutes the multi-dimensional images obtained through the use of ultrasonic imaging.
More specifically, ultrasonic images typically are generated from echoes of transmitted ultrasonic imaging pulses at a frequency of between 500,000 Hz to 15 MHz. The speed of ultrasonic waves in the body is on the order of 1,540 meters per second. The time between the generation of each pulse and its echo is used to determine the distance from the source of the pulse to the source of the echo. Rapid generation of these pulses permits the interrogation of an entire region to build a detailed image.
FIG. 1 displays a prior art ultrasonic imaging system 10. At the front end of the system is a scanhead 20 having a linear array of transducer elements 30 coupled to associated time gain control amplifiers 40 and directed by a beamformer 50. These devices are responsible for selectively generating the ultrasonic imaging pulses, transmitting them to the patient, receiving the echoes returned, amplifying the returned echo signals as appropriate, and combining signals corresponding to the echoes to effectively form beams focused to selected regions of the patient""s body. Once the signals representing the echoes are received and amplified, the signals are processed by a signal/image processing unit 60. The signal/image processing unit 60 receives the amplified echo signals and assembles them into an image of the patient""s internal anatomy. Finally, the image formed by the signal/image processing unit 60 is presented on a display 70. The system might include additional devices, such as storage devices and possibly other output devices (not shown). These supplemental devices allow the results of the scans to be stored and reviewed at a later time. The system 10 also includes a compression map select processor 80, the function of which will be subsequently explained.
This is a simplified rendering of the ultrasonic imaging process; there are many problems that must be overcome for the ultrasonic system to generate a useful image having sufficient resolution to help a medical professional assess the portion of the patient""s body being studied. Some of these problems can be addressed by actually manipulating the patient. For example, because ultrasonic waves do not penetrate gaseous regions well, it is difficult to image any structure behind or beneath a lung or an empty gastrointestinal tract. A physician can mitigate the problem of an empty gastrointestinal tract by requiring the patient to consume a significant amount of fluid without eliminating until after the imaging has taken place.
Other problems, however, are not quite so easily solved. Of these, perhaps the largest single problem is that of noise. In any system, the signal of interest subsists against a background of ambient signals. These other signals have nothing to do with the signal of interest, other than the fact that these unwanted signals interfere with the signal of interest. These ambient signals constitute a noise component. Furthermore, some external noise sources, such as electrical equipment used proximally to the ultrasonic imaging equipment, may introduce noise into the signal path. In addition, as a result of electrons moving through the components of the imaging system itself, the imaging system will exhibit thermal noise. For example, the noise level varies with the phasing of the transducer elements 30 of the scanhead 20 used to generate and direct ultrasonic pulses or receive the echoes of those pulses.
Both the noise and the desired signals are detected by the ultrasonic imaging system 10. Unfortunately, if the noise were to be analyzed as though it were part of the desired signal, the resulting ultrasonic image would be compromised, and the image then would inaccurately portray structures in the patient""s anatomy. To avoid the noise unduly compromising the integrity of the desired signals, the desired signals must be separated from the noise, or the noise must be suppressed as much as practical. There are several mechanisms for reducing noise present in the image, including filtering the received spectrum to consider only frequencies of interest. To the extent noise exists within the spectra of the frequencies of interest, however, frequency filtering does little or nothing to separate the noise from the signals of interest.
Another way to preserve the integrity of the signal, when the magnitude of the desired signals exceeds that of the noise, is to adjust the dynamic range used in mapping the signal to the display 70 of the system 10 through the use of a mapping function referred to as a compression map. As is well known, the dynamic range is an expression of the ratio of the received magnitudes of the largest signal to that of the smallest discernible signal. Dynamic range also commonly known as the signal-to-noise ratio. Dynamic range is typically expressed as a logarithm of a ratio, expressed in units of decibels (dB). Reducing or compressing the dynamic range, in effect, involves cutting off signals having a magnitude below a predetermined value. The dynamic range can be compressed by programming the signal/image processing unit 60 to disregard signals having a magnitude below a certain level so that these unwanted signals do not unduly compromise images shown on the display 70. If the magnitude of the useful component of the signals conveying information about the patient""s tissues is largely greater than the magnitude of the noise component, the noise component can be partially or completely suppressed, leaving a useful signal largely free of noise from which an image can be derived.
However, if the dynamic range is compressed too much, it detracts from the ability to process desired signals. The problem is relatively insignificant when there is relatively little noise present, because suppressing lower magnitude noise signals still allows the system to process desired signals over a larger useable dynamic range. However, as the magnitude of the noise increases, desirable signals will be increasingly affected and xe2x80x9cdrowned outxe2x80x9d by the noise. As the dynamic range is increasingly compressed to avoid the increased noise, unfortunately, desirable signals having magnitudes comparable to the magnitude of the noise also will be lost.
Problems caused by compressing the dynamic range too much become more of a concern as the ultrasonic imaging system 10 targets regions of the patient""s body at increasingly greater distances from the scanhead 20. As is well known in the art, different regions of a patient""s body can be scanned and imaged by manipulating the beamformer 50, which in turn manipulates the transducers 30, the pulses they emit, and the signals they generate, to interrogate regions of the patient""s body at different distances and in different directions relative to the scanhead 20. The coordinates of the region scanned in such a three-dimensional space can be defined by three variables, z, xcex7, and N, where z represents the depth or linear distance from the scanhead 20. As also is known in the art, ultrasonic scans generally are of two forms: sector-type scans, and linear-type scans. In a sector-type scan, the direction of the interrogation is defined by two angles, xcex7 and N, where xcex7 can be regarded is the angular direction of the interrogation in the plane perpendicular to the transducer, and N is the angular direction of the interrogation from a physical axis of the transducer. In a linear-type scan, the direction of the interrogation is defined by an angle xcex7 and a linear dimension N, where xcex7 is the angular direction of the interrogation in a plane perpendicular to the linear motion of the scanhead, and N is a linear displacement from the scanhead. These scanning coordinates, z, xcex7, and N, therefore, determine the three-dimensional position of the region of the patient""s body being interrogated.
FIG. 1B and 1C illustrate how these scanning coordinates define the location of the region of interest. In each figure, equivalent features are numbered identically to clarify the similarities between the figures. FIG. 1B depicts a coordinate system used in a sector-type scan. Corresponding to a ray 12 to the point of interest x 14 from a point of origin 16, in the plane 18 perpendicular to the axis 22 of the transducer, is a projection 24 in the plane 18. Between the projection 24 and a base axis 26 in the plane 18 is the angle xcex7 28, one of the two angular coordinates of the point of interest x 14. From the plane 18 to the ray 12 is the other angular coordinate N 32. The point of interest x 14 is located at a depth z 34 from the point of origin 16. FIG. 1C depicts the coordinate system used in a linear-type scan. Corresponding to the ray 12 to the point of interest x 14 from a point of origin 16, in a plane 44 perpendicular to the linear axis 46 of the transducer, is a projection 48 in the plane 44. Between the projection 48 and the axis of the transducer 22 in the plane 44 is the angle xcex7 52, the angular coordinate of the point of interest x 14. Projecting the ray 12 onto the linear axis 46 is a projection having a length N, one of the linear coordinates of the point of interest x 14. The point of interest 14 again is located at a depth z 34 from the point of origin 16. Therefore, FIGS. 1B and 1C illustrate how the scanning coordinates xcex7, N, define in a sector-type or a linear-type scan, respectively, the location of a point or region of interest.
The problems with overly compressing the dynamic range arise because, as the ultrasonic interrogation is directed to regions at increasingly greater distances from the scanhead 20, as defined by the scanning coordinates xcex7, N, and z, the magnitude of the original ultrasonic signals and the resulting echoes travel through increasingly greater thicknesses of a patient""s tissues. As the signals pass through more and more tissue, the magnitude of the signals becomes attenuated. The time gain control amplifiers 40 apply greater gain to signals representing echoes received from greater distances to compensate for the attenuation in magnitude of these signals. Unfortunately, however, amplifying the signals also amplifies the noise attending the desired signals, and the magnitude of the noise increases. As the time gain control amplifiers 40 apply more and more gain to signals representing echoes from greater and greater distances, the magnitude of the noise increasingly approaches the magnitude of the desired signals. Accordingly, if the dynamic range is further compressed to reject noise of ever-increasing magnitude, lower magnitude components of desired signals will be suppressed along with the noise. Compressing the dynamic range too much, therefore, results in the rejection of desired signals which, like noise, compromises the resulting image.
The problem presented of how compressing the dynamic range too much ultimately begins to eliminate or obscure the desired signal is illustrated by FIG. 2. FIG. 2 is an idealized graph 200 of signal levels, including a noise signal level, as a function of the depth of an ultrasonic interrogation pass. As shown in FIG. 2, the noise level 250 increases with increased depth of the interrogation pass. As was previously described and is well known in the art, time gain compensation amplifiers 40 (FIG. 1) are used to amplify signals representing echoes returned from greater scanning depths to adjust for the attenuation of the emitted ultrasonic pulses and resulting echoes as they pass through increasingly more and more of the patient""s tissues. At depth 210 in the body being interrogated, there is a large dynamic range between the signals with the greatest and smallest magnitudes which can be discerned beyond the noise threshold, because a desirable low magnitude signal of magnitude X 260 still is above the threshold of the noise level 250. However, at depth 220, the threshold of the noise level 250 has increased, and the magnitude of the noise exceeds the that of the desirable signal X 260. As a result, the desirable signal of magnitude of X 260 is enveloped in the noise, and cannot be discerned over the noise. If, at depth 220, the dynamic range was extended to include the desirable signal of magnitude X 260, the noise would compromise the integrity of the data represented by the signals.
Accordingly, the theoretically optimum dynamic range is that which encompasses desirable signals of the lowest possible magnitude down to, but not including, the threshold of the noise level 250. Practically, the optimum dynamic range is that which only slightly overlaps the threshold of the noise level 250, such that the operator of the system can detect a negligible amount of noise to be assured that the dynamic range selected is not so conservative that useable signals are being suppressed. In either case, the system can work with the greatest useful dynamic range to present the best image possible, without the integrity of the image being unduly compromised by the attendant noise. Thus, at 210, the theoretically ideal dynamic range would include the range signified by the line 230. By contrast, at 220, because of the increased noise, a smaller theoretically ideal dynamic range is possible signified by the line 240. Practically, the optimal dynamic range in each case would be slightly greater.
Not surprisingly, noise varies not only with scan depth as in the simplified conception illustrated in FIG. 2, but noise also varies as the ultrasonic imaging system 10 (FIG. 1) interrogates other regions of interest as defined by depth and other changing scanning coordinates. FIGS. 3 and 4 illustrate the degradation in signal to noise ratio with changes in depth and steering angle for a sector-type scan. With reference to FIG. 1B, the graphs of FIGS. 3 and 4 would correspond to a situation where, for example, xcex7 28 remains constant, while N 32 and z 34 change. FIG. 3 shows that, as depth increases, the signal to noise ratio initially improves slightly, but then begins to decrease with increasing depth. The signal to noise ratio degrades because, as depth of the scan increases, additional gain must be applied by the time gain amplifiers 40 (FIG. 1) to make up for attenuation of the ultrasound signals passing through greater thicknesses of a patient""s tissue. Application of additional gain amplifies the noise as well as the desired signal, reducing the useable dynamic range. FIG. 4 shows that the signal to noise ratio, after remaining somewhat level as the steering angle begins to increase, begins to degrade more sharply as the angle becomes more and more oblique. Again, as a result of the steering angle becoming more extreme, additional gain must be applied to compensate for signal attenuation, which amplifies the noise and reduces the useable dynamic range. Therefore, as FIGS. 3 and 4 show, to process signals to take advantage of the greatest useable dynamic range, the dynamic range would have to be adjusted for different regions of the patient being scanned.
To exploit the largest useable dynamic range, the signal/image processing unit 60 (FIG. 1) of the ultrasonic imaging system 10 can be programmed to select a preprogrammed dynamic range for different regions of the patient""s anatomy as a function of changing scanning coordinates. This technology is described in U.S. Pat. No. 5,993,392, entitled xe2x80x9cVariable Compression of Ultrasonic Image Data with Depth and Lateral Scan Dimensions,xe2x80x9d which is incorporated herein by reference. The system described adjusts the dynamic range used in processing the signals resulting from the ultrasonic echoes in response to varying noise levels expected at different regions in the patient""s anatomy relative to the scanhead 20 as a function of changing scanning coordinates. Specifically, the system employs a compression map select processor 80 referenced by the signal/image processing unit 60 to select the estimated optimal usable dynamic range corresponding to the region of the patient presently being interrogated as a function of the scanning coordinates identifying that region. The compression map is created based on empirical evidence from prior scans to predict the level of noise expected at various regions of the patient""s anatomy as defined by the scanning coordinates.
FIG. 5 shows a sample compression map 500 of the type provided by the system described in U.S. Pat. No. 5,993,392. The compression map 500 shows the dynamic range to which the actual range of received signals is to be compressed as a function of changing depths and lateral dimensions for a linear-type scan. The compression map 500 is based on empirically-derived estimates of what noise is expected with changes in these scanning variables. The central region 510 of the compression map 500 shows how the dynamic range varies with changing scan depth within a certain centralized range. For example, from the most shallow depth to the greatest depth, the dynamic range changes from 65 dB, at a depth of 0 cm to 0.5 cm, to 60 dB at a depth of 2.5 cm to 3.5 cm. However, outside the central range of the patient""s body, for which the appropriate dynamic range is indicated by the central region 510 of the compression map 500, different dynamic ranges are used. The appropriate dynamic ranges for these linear extremes is depicted in the outlying regions 520 of the compression map 500. In these outlying regions 520 of the compression map, the dynamic range changes from 60 dB, at a depth of 0 cm to 0.5 cm, to 55 dB, at a depth of 2.5 cm to 3.5 cm. The compression map, therefore, allows for the system to compress the dynamic range as little as possible as different regions of the patient""s anatomy are being interrogated, while still suppressing compromising noise signals. A similar compression map can be constructed for a sector-type scan, adapting the dynamic range for varying scanning depths and steering angles. Returning to FIG. 1, the compression map select processor 80 is used to incorporate the projected maximum available dynamic range for regions as defined by the current scanning coordinates, and compress the dynamic range used to that level.
However, the ultrasonic imaging system 10 shown in FIG. 1 does not necessarily optimize the actual available dynamic range in all cases, because the compression map 500 (FIG. 5) employed by the compression map select processor 80 (FIG. 1) dictates to what dynamic range the actual signals should be compressed under typical imaging conditions. Because it is a predictive technology which compresses the dynamic range based on a typical, expected noise level, it does not adjust the dynamic range when the actual noise level exceeds or falls short of the predicted level. Thus, the scans of some patients will return signals having more or less noise than generally anticipated for scans at the region defined by the scanning coordinates. The atypical magnitude of noise might be as a result of the external conditions, such as other electrical equipment used proximally to the ultrasonic imaging equipment, as a function of thermal noise generated within the ultrasonic imaging system itself, or as a result of other unforeseen factors. As a result, if the predicted available dynamic range is too great or too small, then the noise will cause the system to present a noise-compromised image or unnecessarily restrict the potential clarity of the ultrasonic images, respectively. Naturally, it is best to set the preprogrammed dynamic range conservatively so that the noise impinges only on the periphery of that dynamic range. Unfortunately, the obvious disadvantage to this conservative preprogramming is that potentially useablexe2x80x94and usefulxe2x80x94dynamic range goes unused, resulting in a scan having a resolution less than what it ideally could have.
There is therefore a need for an ultrasonic imaging system that can maximize the available dynamic range for the ultrasonic imaging of any particular patient.
The present invention is directed to a system and method that improves the clarity of images generated by ultrasonic imaging techniques by maximizing the available dynamic range applied in processing the signals. The present invention maximizes the available dynamic range based not just on the noise expected at particular regions of the patient""s anatomy, but on the actual noise detected. According to one aspect of the present invention, actual noise present in particular regions of the patient""s anatomy is sensed by passively monitoring the noise detected. From the noise actually detected in the different regions of the patient""s anatomy, an actual noise function is generated. Then, optionally using a low pass filter to created a smoothly-varying function of measured noise function, this actual noise functionxe2x80x94instead of an expected noise functionxe2x80x94is used to select the greatest available dynamic range to generate the clearest possible ultrasonic images.